Language is inferred or set as part of a user’s device or web browser configuration and I’m afraid I’m not able to comment on volumes related to the public stream as I’m simply unaware. In principle I can imagine that more Tweets may have a language tag or value than the low ~2% where the authors explicitly add location. I’m unable to comment on the specific volumes sorry.

The Gnip APIs are enterprise commercial paid products so be aware that they represent a major investment compared to the free APIs. That said, that would be the way to go for much wider tracking of terms, languages and locations.